Polyesters are widely used in the manufacture of fibers, molded objects, films, sheeting, food trays, as well as food and beverage containers. These polymers are generally made by batch or continuous melt phase polycondensation reactions well known in the art. The polymers are then pelletized and used in various extrusion or molding operations. In certain applications where higher molecular weight polymers are required, the pellets are subjected to "solid state" polycondensation conditions in which the inherent viscosity (I.V.) value is significantly increased. Such solid state polycondensation reactions are used for two reasons. First, because the melt viscosity of polyester polymers is quite high for polymers having I.V. values greater than about 0.6, solid stating provides a convenient means to handle the polymer. Secondly, the solid stating process provides conditions conducive to removing unwanted volatile impurities such as acetaldehyde which is important in some applications as discussed below. Also, polyesters are well known to be degraded by small amounts of moisture when they are melt processed in conventional equipment. Consequently, polyesters are usually carefully dried to very low moisture levels in a drier prior to melt processing. The drying process may remove some objectional volatile materials other than water also.
During the preparation and processing of polyesters such as poly(ethylene terephthalate)(PET) in the melt phase, certain byproducts are formed. One such byproduct is acetaldehyde, and its presence in molded objects such as food containers, beverage bottles, water bottles, and the like is quite deleterious from a taste standpoint. Particularly for sensitive beverages such as cola, beer, and water, it is highly desirable to produce preforms having less than about 10 ppm of acetaldehyde. Achieving this low level of acetaldehyde is difficult, however, because, as is well known to practitioners of the art, acetaldehyde is continually formed as a byproduct during the polymerization and subsequent melt processing of PET and similar polymers.
Before the discovery of the present invention, therefore, a four-stage process has been universally used to provide polyester polymers suitable for uses in which it is important to minimize the presence of acetaldehyde. Such a process typically involves the preparation of a relatively low molecular weight precursor polymer, having an I.V. value of about 0.3-0.6, by melt-phase polymerization techniques that are well known in the art. The acetaldehyde content of such a precursor may range from about 30 ppm to over 150 ppm, depending on the reaction conditions chosen. This precursor is then cooled, shaped into pellets, crystallized, and subjected to further solid-state polymerization at a lower temperature. Typically, a gas is used to strip glycols, acetaldehyde, and other reaction byproducts from the pellets so that at the end of the solid-state process, the I.V. value has been increased to about 0.75 or more, and the acetaldehyde content has been reduced to below about 1 ppm or less. After solid stating, polyesters are commonly handled in contact with ambient air from which it absorbs moisture. Thus, as a third step, the polymer is usually dried immediately prior to the fourth step in which it is heated and melted in order to be formed into a useful shape, such as a beverage bottle preform. The processing typically causes a small decrease in the I.V. of the polymer and an increase in acetaldehyde content of from less than 1 ppm in the pellets, to up to about 8 or 10 ppm or more in the shaped article. This dramatic increase in acetaldehyde occurs despite the fact that the molding process takes typically less than one or two minutes to complete.
We have now discovered a process whereby polyesters such as PET and similar polymers may be prepared and used without need for the solid stating process nor the usual drying of solid pellets. The polymer is prepared and used by a combination of the following operations: melt polymerization, pelletizing, optionally crystallizing, remelting, and forming into useful articles, wherein the polyester is devolatilized during or following polymerization in the melt, while being remelted, after remelting, or any combination thereof. A preferred combination of operations would be to prepare the polymer in the melt to the desired I.V., pelletize the polymer, store and/or transport the amorphous polymer pellets, melt the polymer in a machine designed to dry the polymer while melting it or shortly after melting therein preserving the I.V. of the polymer, devolatilize the melt of acetaldehyde, and forming the purified polymer melt into useful shaped articles, such as for example beverage bottle preforms, wherein the shaped articles having surprisingly low acetaldehyde content. Not only does the process of the present invention avoid the costly additional steps of the conventional process of drying, crystallizing, and solid-state polymerization, but the shaped articles produced by our process possess in addition to low acetaldehyde content, other superior properties such as for example better color, less loss of molecular weight due to breakdown, and freedom from defects known as "bubbles" and "unmelts" which are sometimes formed during the conventional molding process. These and other advantages of the present invention will become apparent in the description which follows.
We are not aware of any prior art which describes the full process of this invention. The following documents may be of interest with respect to certain aspects of the invention.
U.S. Pat. No. 4,340,721 describes PET copolyesters containing 1.5-7.5 mol % of modifying dibasic acids or glycols which have an acetaldehyde content of less than 1.25 ppm.
Japan Patent Application 53-71162 (1978) describes remelting polyester chips and holding the molten polymer under vacuum to reduce the concentration of acetaldehyde.
U.S. Pat. No. 4,263,425 describes the solid stating of PET pellets to provide polymer having a low concentration of acetaldehyde. This reference also states: "Acetaldehyde can partially be eliminated if a stirred melt is treated at higher temperatures under vacuum. By this method, the tolerable minimum limits (of acetaldehyde) are not reached . . . ".
U.S. Pat. No. 4,064,112 describes a method for overcoming sticking problems during the solid stating process. It discusses the disadvantages of a solely melt phase process and states that "elevated concentrations of acetaldehyde are to be expected in the melt."
U.S. Pat. No. 4,362,852 describes devolatilizing molten polyamide or polyester polymers with a rotary disk processor. The disk pack is located very close to the spin block in order to minimize polymer degradation during spinning of fibers.
U.S. Pat. No. 4,836,767 describes a method to reduce acetaldehyde during molding. It states that acetaldehyde increases linearly with time and exponentially with temperature.
Japan Patent Application 55-069618 (1980) states that PET with an acetaldehyde content less than 20 ppm is obtained by melt polymerization followed by extrusion into fiber or film and subsequently passing the fiber or film through a fluid or vacuum. Fluids used included air, nitrogen, water and steam.
U.S. Pat. Nos. 5,119,570 and 5,090,134 mention the necessity of solid stating PET polymers in order to obtain low acetaldehyde concentrations.
U.S. Pat. No. 4,963,644 describes the various reasons for solid stating of PET polymers.
U.S. Pat. No. 4,591,629 states that melt phase produced PET has an unacceptably high level of acetaldehyde and uses solid stating in the presence of water to produce low levels of acetaldehyde.
U.S. Pat. No. 4,230,819 describes the removal of acetaldehyde from crystalline PET with a dry gas (air or nitrogen at 170.degree.-250.degree. C.). It states that acetaldehyde cannot be completely removed from PET by heating it under reduced pressure.
It has also been disclosed in the art that additives may be used for reducing the acetaldehyde levels in PET.
U.S. Pat. No. 5,102,594 describes supplying a thermoplastic condensation polymer such as PET in powder form to a vented extruder in which the polymer is devolatilized and then melted.
U.S. Pat. No. 4,980,105 describes the devolatilization of polycarbonates in an extruder to remove volatiles (especially cyclic dimer) and then the melt is forced through a die.
U.S. Pat. No. 4,255,295 describes a process for reclaiming waste polymer.
U.S. Pat. No. 3,486,864 describes a polymerization reactor in which a solid prepolymer is first melted and then a vacuum is used to remove volatile glycol products as fast as possible. Alternatively, it is suggested that a gas be mixed with the prepolymer prior to heating and melting to entrain the liberated glycol during polycondensation.
U.S. Pat. No. 3,913,796 describes a vent-type injection molding machine in which gases such as moisture, air, and other volatiles can be effectively removed. An extrusion screw is used for heating the solid resin to a semi-molten state prior to the injection molding machine.
U.S. Pat. No. 4,060,226 describes a vented injection molding screw extruder, with means to vent gases and vapors from the screw barrel, to produce devolatilized plasticized materials such as nylon and other degradable materials. Oxygen is excluded by means of a check valve.
U.S. Pat. No. 4,142,040 discloses a method of processing in the molten state a saturated polyester resin so as to minimize degradation to yield acetaldehyde. This patent discloses in column 4, lines 38 et seq., "inert gas is introduced through one or more conduits 3 into the bottom of the hopper or through one or more conduits 3a into the feeding zone (or both). The inert gas flushes essentially all air from the polyester as it advances through the initial part of the feeding zone."